Atlas (C1)

The atlas is the first cervical vertebra, commonly called C1. It is an atypical cervical vertebra with unique features. It articulates with the dens of the axis and the occiput, respectively allowing rotation of the head, and flexion, extension and lateral flexion of the head.  Unlike the rest of the cervical vertebrae, with exception to the similarly structured axis, the anterior rami are sent posteriorly instead of anteriorly. 

Gross anatomy

The atlas is composed of an anterior arch and a posterior arch, paired lateral masses, and paired transverse processes. It does not have a vertberal body, instead the dens of the axis sit where a centrum (body) of a typical vertebra would be. The transverse ligament holds the dens of the axis against the anterior arch of the atlas and divides its vertebral canal into two parts. The anterior 1/3 is occupied by the dens. The posterior 2/3 contains the spinal cord, which occupies 1/3 of the total vertebral canal space.

• anterior arch
  • anterior tubercle: sits on the anterior aspect of the anterior arch and is the site of attachment of the anterior longitudinal ligament
  • posterior facet for the dens: sits on the posterior aspect of the anterior arch
  • upper border: attachment of the anterior atlanto-occipital membrane and lateral parts of the anterior longitudinal ligament
  • lower border: attachment of the anterior atlanto-occipital membrane and lateral parts of the anterior longitudinal ligament
• posterior arch
  • 3/5th of circumference of the ring
  • posterior tubercle: sits posteriorly toon the posterior aspect of the posterior arch, is a rudimentary spinous process and attachment site for the ligamentum nuchae
  • superior surface: contains paired grooves for the C1 nerve  and vertebral artery, sits just posterior to the lateral mass
  • superior border: attachment for the posterior atlanto-occipital membrane
  • inferior border: attachment for the ligamentum flavum
• lateral masses
  • paired, ovoid
  • superior articular facet: kidney-shaped, concave and articulates with the occipital bone
  • inferior articular facet: circular, with a flat or slightly concave surface articulating with the lateral atlanto-axial joint
  • medial surface: marked by vascular foramina and a tubercle for the attachment of the transverse ligament
• transverse processes
  • longer than all of the transverse processes of the cervical vertebrae except C7
  • typically covered by costal lamella 
  • transverse foramina: contains the vertebral arteries
  • anterior tubercle: sometimes present on the anterior aspect of the transverse process

Articulations

• atlanto-occipital joint:  hyaline-covered synovial joint between the occipital condyle and concave facet of the lateral mass of the atlas. Covered by a capsule and innervated by C1, this joint allows for flexion, extension and lateral flexion.
• median atlanto-axial joint: hyaline-covered synovial joint between the dens of the axis and the posterior aspect of the anterior arch of the atlas, allowing for the rotation of the head. The dens is held in place by the transverse ligament, with a bursa between the two. 
• lateral atlanto-axial joint: hyaline-covered synovial joint between the inferior articular facet of the atlas and the superior articular facet of the axis which allows for the rotation of the head. A capsule innervated by the C2 nerve surrounds the joint.

Ligamentous

• transverse ligament: strong band that runs posterior to the dens of the axis, holding it in place. Each end is attached to tubercles on the anterior arch of the atlas.

• atlanto-axial ligaments: attach from the lower border of the anterior arch of the atlas to front of the body of the axis. Provides tertiary support against ventral translation of the dens.

Musculotendinous

• anterior atlanto-occipital membrane: attached to upper border of the anterior arch to the outer margins of foramen magnum
• posterior atlanto-occipital membrane: attached to upper border of the posterior arch to the outer margins of foramen magnum. At each lateral margin, there is a gap for the passage of the C1 nerve and vertebral artery, which sometimes ossifies and becomebecomes a foramen. Innervated by C1.
• longus colli: deep grooves of the anterior surface of the bodytubercle
• levator scapulae: tip of the transverse processes
• splenius cervicis: transverse processes
• obliquus capitis superior: transverse processes
• obliquus capitis inferior: transverse processes 
• rectus capitis posterior minor: tubercle on the posterior arch of the atlas

Variant anatomy

• arcuate foramen: calcification of the posterior atlanto-occipital membrane
• superior articular facets of the lateral masses can be divided into two parts, with the anterior part being larger and the posterior smaller
• central part of the posterior arch can be absent and replaced by fibrous tissue
• fusion (with the occiput - partially or fully)

Radiographic features

• plain radiograph: transoral AP view best appreciates the atlanto-axial joint
  • anterior atlanto-dental interval: up to 3 mm ventral translation is normal ⁶
  • sum of displacement of lateral masses of C1 compared to C2 > 8 mm on transoral AP view ⁶
• CT
• MRI: ligamentous disruption or avulsion 

Development

Ossification starts from three centres: anterior ossification centre, and paired lateral mass ossification centres. The paired lateral mass ossification centres arise in week 7 and extend to the posterior arch. Unification at the posterior arch occurs at years 3-4 and is typically direct, but can sometimes involve a third centre at the posterior arch. The anterior ossification centre unites with the lateral mass ossification centre at years 6-8. There are sometimes two anterior ossification centres.

History and etymology

The name "atlas" is derived from the Greek god who bore the world on his shoulders ³.

Related pathology

• Classification of atlas fractures ⁶
  • type I: isolated posterior arch fracture
  • type II: isolated anterior arch fracture
  • type III (Jefferson fracture): anterior and posterior arch fractures 
  • type IV: comminuted lateral mass fracture
• atlanto-occipital dissociation injuries
• atlanto-axial subluxation